Conventional backlight units have consisted of a cold cathode fluorescent lamp (CCFL) and a diffuser sheet to give large areas of homogenous white light. Due to energy and size constraints more recently RGB-LEDs have replaced the CCFL light source (FIG. 1). A further development has been to use a blue LED excitation source in combination with a sheet containing a conventional phosphor, such as YAG, whereby the “phosphor layer” or “phosphor sheet” is located near or on top of the diffuser layer and away from the light/excitation source (FIG. 2).
Currently phosphorescent materials used in down converting applications, absorb UV or mainly blue light and convert it to longer wavelengths, with most phosphors currently using trivalent rare-earth doped oxides or halophosphates. White emission is obtained by blending phosphors which emit in the blue, green and red regions with that of a blue or UV emitting solid-state device. i.e. a blue light emitting LED plus a green phosphor such as, SrGa2S4:Eu2+, and a red phosphor such as, SrSiEu2+ or a UV light emitting LED plus a yellow phosphor such as, Sr2P2O7:Eu2+; Mu2+, and a blue-green phosphor.
Presently white LEDs are made by combining a blue LED with a yellow phosphor however, colour control and colour rendering is poor when using this methodology due to lack of tunability of the LEDs and the phosphor. Moreover, conventional LED phosphor technology uses down converting materials that have poor colour rendering (i.e. colour rendering index (CRI)<75) due to the lack of available phosphor colours.
There has been substantial interest in exploiting the properties of compound semiconductors consisting of particles with dimensions in the order of 2-50 nm, often referred to as Quantum Dots (QDs) or nanocrystals. These materials are of commercial interest due to their size-tuneable electronic properties which can be exploited in many commercial applications such as optical and electronic devices and other applications that now range from biological labeling, photovoltaics, catalysis, light-emitting diodes, general space lighting and electroluminescent displays amongst many new and emerging applications. Two fundamental factors, both related to the size of the individual semiconductor nanoparticle, are responsible for their unique properties. The first is the large surface to volume ratio; as a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. This leads to the surface properties playing an important role in the overall properties of the material. The second factor, which affects many materials including semiconductor nanoparticles, is a change in the electronic properties of the materials with size; because of quantum confinement effects the band gap gradually becomes larger as the size of the particle decreases. This effect is a consequence of the confinement of an ‘electron in a box’ giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than a continuous band as observed in the corresponding bulk semiconductor material. Thus, for a semiconductor nanoparticle, because of the physical parameters, the “electron and hole”, produced by the absorption of electromagnetic radiation, a photon, with energy greater than the first excitonic transition, are closer together than they would be in the corresponding macrocrystalline material; moreover the Coulombic interaction cannot be neglected. This leads to a narrow bandwidth emission, which is dependent upon the particle size and composition of the nanoparticle material. Thus, QDs have higher kinetic energy than the corresponding macrocrystalline material and consequently the first excitonic transition (band gap) increases in energy with decreasing particle diameter.
Core semiconductor nanoparticles, which consist of a single semiconductor material along with an outer organic passivating layer, tend to have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface which can lead to non-radiative electron-hole recombinations.
One method to eliminate defects and dangling bonds on the inorganic surface of the QD is to overcoat the nanoparticles with a homogeneous shell of a second semiconductor. This semiconductor material typically has a much wider band-gap than that of the core to suppress tunnelling of the charge carriers from the core to the newly formed surface atoms of the shell. The shell material must also have a small lattice mismatch to that of the core material. Lattice mismatch arises primarily because of the differences in bond lengths between the atoms in the core and in the shell. Although the differences in the lattice mismatch between the core and shell materials may only be a few percent it is enough to alter both the kinetics of shell deposition and particle morphology as well as the QY of the resultant particles. Small lattice mismatch is essential to ensure epitaxial growth of the shell on the surface of the core particle to produce a “core-shell” particle with no or minimum defects at the interface that could introduce non-radiative recombination pathways that reduce the PLQY of the particle. One example is a ZnS shell grown on the surface of a CdSe or InP core. The lattice mismatch of some of the most common shell materials relative to CdSe is 3.86% for CdS, 6.98% for ZnSe and 11.2% for ZnS.
Another approach is to prepare a core-multi shell structure where the “electron-hole” pair is completely confined to a single shell layer consisting of a few monolayers of a specific material such as a QD-quantum well structure. Here, the core is of a wide band gap material, followed by a thin shell of narrower band gap material, and capped with a further wide band gap layer, such as CdS/HgS/CdS grown using substitution of Hg for Cd on the surface of the core nanocrystal to deposit just a few monolayers of HgS which is then over grown by a monolayer of CdS. The resulting structures exhibit clear confinement of photoexcited carriers in the HgS layer, which result in a high PLQY and improved photochemical stability.
To add further stability to QDs and help to confine the electron-hole pair one of the most common approaches is to grow thick and robust shell layers around the core. However, because of the lattice mismatch between the core and shell materials, the interface strain accumulates dramatically with increasing shell thickness, and eventually can be released through the formation of misfit dislocations, degrading the optical properties of the QDs. This problem can be circumvented by epitaxially growing a compositionally graded alloy layer on the core as this can help to alleviate the strain at the core-shell interface. For example in order to improve the structural stability and quantum yield of a CdSe core, a graded alloy layer of Cd1-xZnxSe1-ySy can be used in place of a shell of ZnS directly on the core. Because of the gradual change in shell composition and lattice parameters the resulting graded multi-shell QDs are very well electronically passivated with PLQY values in the range of 70-80% and present enhanced photochemical and colloidal stability compared to simple core-shell QDs.
Doping QDs with atomic impurities is an efficient way also of manipulating the emission and absorption properties of the nanoparticle. Procedures for doping wide band gap materials, such as zinc selenide and zinc sulfide, with manganese and copper (ZnSe:Mn or ZnS:Cu), have been developed. Doping with different luminescence activators in a semiconducting nanocrystal can tune the photoluminescence and electroluminescence at energies even lower than the band gap of the bulk material, whereas the quantum size effect can tune the excitation energy with the size of the QDs without having a significant change in the energy of the activator related emission. Dopants include main group or rare earth elements, often a transition metal or rare earth element, such as, Mn+ or Cu2+.
The coordination around the atoms on the surface of any core, core-shell or core-multishell, doped or graded nanoparticle is incomplete and the non-fully coordinated atoms have dangling bonds which make them highly reactive and can lead to particle agglomeration. This problem is overcome by passivating (capping) the “bare” surface atoms with protecting organic groups.
The use of QDs in light emitting devices has some significant advantages over the use of the more conventional phosphors such as the ability to tune the emission wavelength, strong absorption properties and low scattering if the QDs are mono-dispersed. However the methods used so far are challenging due to chemical incompatibility between the outer organic surfaces of the QDs and the types of host materials in which the QDs are supported. QDs can suffer from agglomeration when formulating into these materials and, once incorporated, can suffer from photo-oxidation as a result of the migration of oxygen through the host material to the surfaces of the QDs, which can ultimately lead to a drop in quantum yield. Although reasonable devices can be made under laboratory conditions, there remain significant challenges to replicate this under commercial conditions on a large scale. For example, at the mixing stage the QDs need to be stable to air.
Devices incorporating a light emitting layer where semiconductor QDs are used in place of the conventional phosphors have been described, however, due to problems relating to processability and the stability of the QD-containing materials during and after layer fabrication, the only types of QD material that have been successfully incorporated into such layers are relatively conventional II-VI or IV-VI QD materials, e.g. CdSe, CdS and PbSe. Cadmium and other restricted heavy metals used in conventional QDs are highly toxic elements and represent a major concern in commercial applications. The inherent toxicity of cadmium-containing QDs prevents their use in any applications involving animals or humans. For example recent studies suggest that QDs made of a cadmium chalcogenide semiconductor material can be cytotoxic in a biological environment unless protected. Specifically, oxidation or chemical attack through a variety of pathways can lead to the formation of cadmium ions on the QD surface that can be released into the surrounding environment. Although surface coatings such as ZnS can significantly reduce the toxicity, it may not completely eliminate it because QDs can be retained in cells or accumulated in the body for a long period of time, during which their coatings may undergo some sort of degradation exposing the cadmium-rich core.
The toxicity affects not only the progress of biological applications but also other applications including optoelectronic and communication because heavy metal-based materials are widespread in many commercial products including household appliances such as IT & telecommunication equipment, lighting equipment, electrical & electronic tools, toys, leisure & sports equipment. A legislation to restrict or ban certain heavy metals in commercial products has been already implemented in many regions of the world. For example starting 1 Jul. 2006, the European Union directive 2002/95/EC, known as the “Restrictions on the use of Hazardous Substances in electronic equipment” (or RoHS), banned the sale of new electrical and electronic equipment containing more than agreed levels of lead, cadmium, mercury, hexavalent chromium along with polybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE) flame retardants. This law required manufacturers to find alternative materials and develop new engineering processes for the creation of common electronic equipment. In addition, on 1 Jun. 2007 a European Community Regulation came into force concerning chemicals and their safe use (EC 1907/2006). The Regulation deals with the Registration, Evaluation, Authorisation and Restriction of Chemical substances and is known as “REACH”. The REACH Regulation gives greater responsibility to industry to manage the risks from chemicals and to provide safety information on the substances. It is anticipated that similar regulations will be extended worldwide including China, Korea, Japan and the US.
There are currently no light emitting layers available that contain heavy metal-free QDs, which can be fabricated at commercially feasible cost and that emit light efficiently in the visible spectrum.